SEM-EDS and XRD study of a NiCrAlU alloy at temperatures from 800∘^{\circ}C to 1200∘^{\circ}C

The oxidation behaviour of a nominally 74.5wt%Ni, 20wt%Cr, 5wt%Al and 0.5wt%U alloy was examined using Scanning Electron Microscopy with X-ray Energy Dispersive Spectroscopy (SEM-EDS) and X-ray Diffraction (XRD). The oxidation temperature ranged from 800$^{\circ}$C to 1200$^{\circ}$C in air, with oxidation times of up to 120 h. Analysis showed that during short term oxidation (40 h), at the lower temperatures, oxides of Cr, Ni and Al were formed. With increasing oxidation time, the Cr-rich oxide scale became predominant with only a few regions of Ni-rich oxide being observed. No uranium was detected, using EDS, within the outer scale. The scale formed was normally flat but with spallation occurring after 40 h oxidation at 900$^{\circ}$C, with more spallation and some convolution being observed with increasing temperature and time. 
Cross-sectional investigations indicated the presence of an internal oxidation zone comprising an Al-rich oxide, with significant amounts of uranium, beneath the outer Cr-rich scale. The depth of this zone varied from 1 micron to 60 microns and the depth increased with increasing oxidation temperature and time. At higher temperatures (above 1000$^{\circ}$C), the internal oxidation zone ceased to grow with longer oxidation times, due to the internal oxides forming a complete layer of alumina which protected the metal substrate from further degradation. The compositions of the scales formed reflect the selective oxidation of chromium and aluminium as a result of their higher reactivity compared to nickel and the greater stability of Al$_{2}$O$_{3}$ and Cr$_{2}$O$_{3}$ compared to NiO.
This study showed that Al-rich oxide was not the predominant protective outer scale, but instead formed as an internal oxide. The outer scale was predominantly Cr-rich oxide and for most samples was flat and adherent, although a few regions were observed to convolute. This shows that uranium influences the growth morphology of the Cr-rich scales formed on NiCrAl and that it acts as a reactive element, even though uranium was only detected at the metal grain boundaries and within the internal oxidation zone. The concentrations of uranium required to change the scale morphology appear to be lower than can be detected using SEM/EDS

FEG-SEM investigation of α\alpha-alumina scales formed on FeCrAlY alloys oxidised at 1200∘^{\circ}C

This work is part of an ongoing European funded project, “SMILER”, with the aim of improving the performance of alumina forming Fe-20Cr-5Al-Y alloys for high temperature industrial applications. One aspect of the project is to investigate the influence of additives on the oxidation behaviour of these alloys. During this study a LEO 1550 FESEM (field emission scanning electron microscope), equipped with INCA X-ray microanalysis facilities was used. Ultra-high-purity model alloys, where the levels of additives (Hf, Zr, Ti, Si, La and Y) were carefully controlled, and two commercial Aluchrom YHfAl and Kanthal AMPT alloys were oxidised at 1200$^{\circ}$C for up to 3100h (100h/cycle).
The YHfAl, (Y+Ti+Zr)- and (Y+Zr+Hf)-containing alloys showed the highest oxidation rates when oxidised, whereas the La-containing alloy showed the lowest oxidation rate. However, the La-containing alloy spalled the most, while the (Y+Zr+Hf)-containing and YHfAl alloys showed little spallation, and the additives appeared to have a major influence on the spallation of the $\alpha$-alumina scale formed. On the alloys with La and Si respectively, the scale spalled at the scale/metal interface (adhesively), whereas on alloys containing (Y+Hf+Ti) and (Y+Hf+Zr) and the YHfAl alloy, the scale spalled mainly in a cohesive manner (within the scale). Also, the added elements affected the scale topography. In general the scale had a columnar structure at the scale/metal interface, whereas the grains were equiaxed at the scale/gas interface. However, in the case of YHfAl and (Y+Zr+Hf)-containing alloys, a sunflower type structure was observed in both fractured samples, in the regions where the scale spalled cohesively. Preliminary EDX analyses revealed that, in the case of YHfAl, for example, the center of the sunflower structure was rich in Mg. This suggests that there may be inhomogeneities in the metal substrate, prior to oxidation, where high concentrations of Mg increase the local oxide growth rate

The effects of milling conditions on the subsequent oxidation behaviour of mechanically alloyed Fe3_{\bf 3}Al-based powders

Mechanically alloyed, Fe$_{3}$Al-based, oxide dispersion strengthened alloys form a surface oxide scale during powder processing. This scale becomes entrained in the consolidated alloy, and may have a significant effect on subsequent recrystallisation behaviour. The high oxidation rates found in these alloys are mainly due to the bulk alloy composition. However, batch-to-batch differences in oxidation mass gain occur in powders with ostensibly identical compositions. Batches PMWY2 and PMWY3 were studied and parameters such as alloy composition and homogeneity, powder surface area to volume ratios and scale thickening rates considered. Batch PMWY2 showed 20-90% faster weight gain than PMWY3 and reached the onset of breakaway oxidation approximately twice as quickly. PMWY2 was found to contain aluminium-depleted regions, whereas PMWY3 is much more homogeneous. The surface area to volume ratio for PMWY2 was 44% higher than that of PMWY3, and batch PMWY2 was found to contain extremely fine powder particles. The scale on batch PMWY2 thickened more quickly than that on batch PMWY3, with rates 20-40% higher at different stages in the oxidation. The major contributory factor to the difference in oxidation mass gain between the two alloy batches is scale thickening rate and factors influencing thickening rates are discussed

Significance of Minor Alloying Additions and Impurities on Alumina Scale Growth and Adherence in FeCrAl Alloys

Ultra-high purity Fe-Cr-Al-Y model alloys with controlled additions of impurities such as phosphorus and carbon, and potentially more beneficial elements such as titanium and zirconium have been prepared by induction melting in water-cooled, silver crucibles. 1 mm thick samples were then prepared by hot and cold rolling and annealing prior to cyclic oxidation in air at temperatures in the range 1100-1300degreesC. Other impurities were kept to a minimum of <10 ppm. scanning electron microscopy, Auger surface analysis and Scanning Transmission Electron Microscopy were used to characterise the samples both before and after oxidation.Weight gain studies during oxidation showed that the high phosphorus containing alloy went into breakaway very quickly, after only 200 hours at 1300degreesC, while the Ti and Zr rich samples lasted for 1900 hours and 3300 hours respectively. In some cases, chromium or titanium rich precipitates were found along the alloy grain boundaries, often associated with carbon, while in other cases precipitates were found along the oxide metal interface. Although phosphorus was found at this interface in some of the samples, it was not always present, and may not be the only contributing factor to the premature breakaway failure of the oxides. A complete review of the microstructural evolution of these samples during oxidation will form the main topic of this paper

The effect of aluminium depletion on the oxidation behaviour of FeCrAl foils

In thin FeCrAl foils, the formation of a chromia layer within or underneath the alumina layer has been observed after consumption of the aluminium from the alloy. For Aluchrom I SE, the growth law of the alumina-forming step has been evaluated and an activation energy for oxygen diffusion of 383 +/- 36 kJ mol(-1) has been determined. For the growth mechanism of the chromia layer, three models are introduced and discussed. In agreement with a model proposed by H. E. Evans, the measurements of the aluminium content resulted in complete aluminium consumption before the beginning of chromia formation. Because of strong deformation of the thin samples during oxidation, a model is proposed to calculate the alloy thickness based on the amount of aluminium consumption. From a comparison of these calculated values with the measured thicknesses, the elongation of the sample due to creep processes could be determined